1. Field of the Invention
This invention relates to a process for coating electrically conductive objects with an anionic polyurethane coating by immersing the object as the anode into an aqueous polyurethane dispersion and passing an electric current between the anode and a cathode which also is immersed in the dispersion.
2. Description of the Prior Art
Electro-immersion coating is an environmentally sound process where the coating is deposited on electrically conductive objects. Following this process, the coatings are cross-linked, generally at increased temperatures.
Radiation curing of coatings applied by spraying, immersion, and brushing is also known in the art. This process requires binders which involve the use of prepolymers containing olefinic groups together with poly- and monofunctional monomers. Environmental protection problems are encountered in processing these mixtures due to the vapor pressure of the monomers used.
However, by combining both processes, it is possible to eliminate the environmental protection problems of the conventional radiation cured coating as well as to produce an electro-immersion coating which need not be thermally cross-linked. This process may be applied in those instances where metal surfaces must be uniformly coated. Conventional application of radiation cured coatings to metals does not result in the uniformity of the layer thickness which is possible with electro-immersion coating. The adhesion to the metallic substrate of the coatings, for instance by spraying, immersion, and brushing, is poor. Electrophoretically deposited paint films adhere considerably better.
German Published Application No. 2,207,031 describes a coating which consists of an electro-immersion primer and a paint coat which can be cured by irradiation and which is applied to the primer. Such a coating is also described in German Application No. 2,015,287 and Japanese Pat. No. 73/04,446. In order to improve the adhesion between the two layers, the multi-layer paint coats must be baked for some time at increased temperatures.
Coatings containing the polar groups required for the electro-immersion deposition, as well as the double bonds required for the radiation curing, are described in German Application No. 2,301,075. The binders used consist of unsaturated acid resins and water-insoluble polyfunctional ultraviolet cross-linking agents. In this context, acid resins are understood to be reaction products of an epoxide resin and unsaturated fatty acids which are maleinized fatty acid-modified unsaturated acid alkyd resins, and maleinized oils which are modified with styrene or methylacrylate. Resins containing ethylenically unsaturated double bonds and carboxyl groups, which can be used in electrophoretic deposition, are also described in U.S. Pat. No. 4,040,925.